The present invention relates to a flex spline used for a meshing type gear device (wave motion gear device) and the meshing type gear device (wave motion gear device) having the flex spline.
The meshing type gear device (wave motion gear device) is conventionally well known which includes a flex spline having an attachment portion formed normally as a boss portion and coupled to an output extraction member, a tubular portion with a thin thickness normally continuing to the attachment portion through a diaphragm and capable of being elastically deformed, and external teeth formed at the tip end portion of the tubular portion away from the attachment portion side; a circular spline having inner teeth which mesh with the external teeth of the flex spline and having the number of teeth slightly larger than that of the external teeth; and a wave generator which is formed in an elliptical shape and engages with the inner side of the tubular portion of the flex spline thereby to flexibly deform the flex spline. In particular, the meshing type gear device called Harmonic Drive (trade name) is well known.
At the time of processing the inner cylinder of the flex spline 1, normally, both the teeth portion 1c of the flex spline 1 and the attachment portion (boss portion) 1a thereof on the opposite side are grasped by a chuck of a lathe etc., and a bite and the flex spline are relatively moved in the shaft line direction while both the tooth portion and the attachment portion are rotated around the shaft lines thereof thereby to perform the cutting process thereof.
To this end, in the conventional flex spline 1, as shown in FIG. 4A, the tubular portion 1b with a thin thickness is formed in a straight cylindrical shape. That is, as shown in FIG. 4A, the inner diameter d1 on the attachment portion (boss portion) 1a side is equal to the inner diameter d2 on the thin-thickness tubular portion 1b side, that is, d1=d2. Each of the inner diameter d1 and the inner diameter d2 is set to be smaller than the pitch diameter d0 of the inner teeth 5a of a circular spline 5 described later.
In such a conventional meshing type gear device (wave motion gear device), usually, the attachment portion (boss portion) 1a is configured to be hardly deformed in order to fix the flex spline 1 and extracting an output torque. On the other hand, in the conventional meshing type gear device, as shown in Japanese Patent Laid-Open No. 17888/1994 and Japanese Utility Model Laid-Open No. 173851/1986, the teeth portion 1c of the flex spline 1 is formed on the thin-thickness tubular portion 1b and the teeth portion 1c on the outer side of the flex spline 1 can be meshed with the teeth portion 5a of the circular spline 5.
The wave generator 3 having a cam formed in an elliptical shape is inserted-through a bearing 4 into the inner diameter portion at the tip end of the tubular portion 1b of the flex spline 1. The tooth portion 1c on the outer side of the flex spline 1 meshes with the circular spline 5 in a manner that the flex spline 1 is deformed by the wave generator 3, and the radius position of the tooth portion 1c of the flex spline 1 at which the major axis of the ellipse of the wave generator 3 is positioned is made positioned on the outer side from the radius position before the deformation.
As described above, the tubular portion 1b of the flex spline 1 is formed in the straight cylindrical shape. Thus, in the flex spline 1 where the wave generator 3 is inserted into the inner diameter portion of the tubular portion, the attachment portion 1a does not deform. In contrast, since the teeth portion 1c of the thin-thickness tubular portion 1b is expanded outwardly as shown in FIG. 4B by means of the wave generator 3, the tooth trace of the flex spline 1 having been deformed does not become in parallel to the tooth trace of the circular spline 5, so that a coning angle α is formed at the teeth portion.
Thus, the meshing depth between the external teeth 1c of the flex spline 1 and the inner teeth 5a of the circular spline 5 differs in the tooth-width direction (that is, the shaft line direction of the flex spline 1). Since the meshing depth differs in the tooth-width direction, the load is not applied uniformly over the entire width of the tooth but applied to a portion of the tooth concentrically. Usually, although each of the external teeth 1c of the flex spline 1 and the inner teeth 5a of the circular spline 5 is a spur tooth, a normal meshing state can not be obtained between these spur external teeth 1c and 5a. 
Further, due to the influence of the aforesaid coning angle α of the flex spline 1, the wave generator 3 is made in contact only on its one end side with the inner diameter portion of the flex spline 1 when viewed along the shaft line direction of the flex spline 1. Thus, the life time of the bearing 4 attached between the wave generator 3 and the flex spline 1 is reduced and the meshing rigidity between the wave generator 3 and the flex spline 1 is also degraded.
Furthermore, even if a roller bearing with a large bearing capacity is tried to be attached between the wave generator 3 and the flex spline 1, due to the aforesaid coning angle α and the one end side contact state of the wave generator caused by the coning angle, the roller of the roller bearing inclines and so the inner wheel and the outer wheel of the bearing are abraded or worn out by the shoulder portion of the roller. Thus, such a rolling bearing can be not be applied.
The shorter the length (the length along the axial direction) of the body portion of the flex spline 1 becomes, the more the aforesaid various problems accompanied by the conventional technique become remarkable.